1. Field of the Invention
The present invention relates to an organic electroluminescent display device, and more particularly, to an organic electroluminescent display device having a pre-charge circuit and a driving method thereof.
2. Discussion of the Related Art
Among flat panel displays (FPDs), organic electroluminescent (EL) devices have been of particular interest in research and development because they are self-light-emitting type displays having a wide viewing angle as well as a high contrast ratio in comparison to liquid crystal display (LCD) devices. Organic EL devices are lightweight and small, as compared to other types of display devices, because they do not need a backlight. Organic EL devices have other desirable characteristics, such as low power consumption, superior brightness and fast response time. When driving the organic EL devices, only a low direct current (DC) voltage is required. Moreover, a fast response time can be obtained.
Unlike LCD devices, organic EL devices are entirely formed in a solid phase arrangement. Thus, organic EL devices are sufficiently strong to withstand external impacts and also have a greater operational temperature range. Moreover, organic EL devices are fabricated in a relatively simple process involving few processing steps. Thus, it is much cheaper to produce an organic EL device in comparison to an LCD device or a plasma display panel (PDP). For example, only deposition and encapsulation processes are necessary for manufacturing organic EL devices. An organic EL device is often referred to as an organic light emitting diode (OLED).
There are two types of organic EL display devices: passive matrix type and active matrix type. While both the passive matrix organic EL display device and the active matrix organic EL display device have simple structures and are formed by a simple fabricating process, the passive matrix organic EL display device requires a relatively high amount of power to operate. In addition, the display size of a passive matrix organic EL display device is limited by its structure. Furthermore, as the number of conductive lines increases, the aperture ratio of a passive matrix organic EL display device decreases.
In contrast, active matrix organic EL display devices are highly efficient and can produce a high-quality image for a large display size with a relatively low power. In general, in an active matrix type organic EL device, a voltage controlling a current applied to a pixel is stored in a storage capacitor. Accordingly, the voltage in the storage capacitor can be applied to the pixel until a next frame and the pixel can continuously display an image during one frame. As a result, an active matrix type organic EL device has a low power consumption, a high resolution and a large display size because it can display images with a constant brightness in spite of a low driving current.
FIG. 1 is a circuit diagram showing an organic electroluminescent display device according to the related art. In FIG. 1, a gate line is disposed along a first direction and a data line is disposed a second direction crossing the gate line. A pixel region is defined by the gate line and the data line. A power line spaced apart from the data line is disposed along the second direction. A switching thin film transistor (TFT) “TS” as an addressing element is connected to the gate line and the data line. A storage capacitor “CST” is connected to the switching TFT “TS.” A driving TFT “TD” as a current source element is connected to the switching TFT “TS,” the storage capacitor “CST” and the power line. An organic electroluminescent (EL) diode “DEL” including first and second electrodes is connected to the driving TFT “TD.” The switching TFT “TS” controls a gate voltage of the driving TFT “TD” and the storage capacitor “CST” stores the gate voltage of the driving TFT “TD.”
During a non-selected time period, a gate scanning pulse is applied to a gate electrode of the switching TFT “TS,” the switching TFT “TS” is turned on and a data signal is transferred to the driving TFT “TD” and the storage capacitor “CST” through the switching TFT “TS.” In addition, the driving TFT “TD” is turned on and a current is supplied to the organic EL diode “DEL” through the driving TFT “TD” by the power line. As a result, an organic luminescent layer of the organic EL diode “DEL” emits light. Since a turn-on ratio of the driving TFT “TD” depends on a magnitude of the data signal, the current passing through the driving TFT “TD” can be adjusted by the data signal to display various degree of gray. Furthermore, during a non-selected time period where the gate scanning pulse is not applied to the switching TFT “TS,” the data signal stored in the storage capacitor “CST” is continuously applied to the driving TFT “TD.” Accordingly, the organic EL diode “DEL” continuously emits light until a gate signal of a next frame is applied to the switching TFT “TS.”
FIG. 2 is a timing chart showing a plurality of driving signals for a four-block driving method of an organic electroluminescent display device according to the related art. As shown in FIG. 2, the four-block driving method uses Nth and (N+1)th gate clocks “GCLKN” and “GCLKN+1,” a data start signal “DVST,” first to fourth data clocks “DCLK1” to “DCLK4” and a data signal “VIDEO.” Gate scanning pulses are sequentially applied to an Nth gate line and an (N+1)th gate line according to the respective gate clock, “GCLKN” and “GCLKN+1,” and switching TFTs connected to the Nth gate line and the (N+1)th gate line are sequentially turned on. When the Nth gate line is selected, the first to third data clocks “DCLK1” to “DCLK3” are sequentially generated. Accordingly, the data signal “VIDEO” supplied to a corresponding data line is transferred to a gate electrode of a corresponding driving TFT through the corresponding switching TFT. Since the data signal “VIDEO” controls the driving TFT, transfer accuracy of the data signal “VIDEO” to an organic EL diode depends on characteristics of the driving TFT.
FIG. 3 is a graph showing characteristics of a driving thin film transistor for an organic electroluminescent display device according to the related art. In FIG. 3, the x-axis and the y-axis respectively represent a gate-source voltage “Vgs” between a gate electrode and a source electrode and a drain-source current “Ids” flowing between a drain electrode and a source electrode. As shown in the I-V curve, in a section of a first voltage value “V1” to a third voltage value “V3,” the drain-source current “Ids” increases as the gate-source voltage “Vgs” increases. Accordingly, the drain-source current “Ids” input to an organic electroluminescent (EL) diode from a power line is adjusted by controlling the gate-source voltage “Vgs,” thereby controlling the organic EL diode to display images of various grays.
However, when the driving TFT has a hysteresis, the driving TFT fails to normally transfer a data signal to the organic EL diode. A first curve 10 is obtained when the drain-source current “Ids” is measured along an increasing “SWEEP1” of the gate-source voltage “Vgs,” while a second curve 20 different from the first curve 10 is obtained when the drain-source current “Ids” is measured along a decreasing “SWEEP2” of the gate-source voltage “Vgs.” Both of the first and second curves 10 and 20 have a first current value “IA” at the third voltage value “V3” corresponding to a white image, and have a fourth current value “ID” at the first voltage value “V1” corresponding to a black image. Accordingly, the white image and the black image may be displayed without difference along the increasing “SWEEP1” of the gate-source voltage “Vgs” and along the decreasing “SWEEP2” of the gate-source voltage “Vgs.”
For the second voltage value “V2” corresponding to a gray image, however, a second current value “IB” measured along the increasing “SWEEP1” of the gate-source voltage “Vgs” is higher than a third current value “IC” measured along the decreasing “SWEEP2” of the gate-source voltage “Vgs.” Accordingly, when the second voltage value “V2” is applied to the gate electrode of the driving TFT, the drain-source current “Ids” of the driving TFT is differently determined according to a gate voltage of the previous frame. Further, such a difference in the drain-source current “Ids” causes a difference in brightness of the organic EL diode.
FIGS. 4A and 4B are schematic views showing images produced by an organic electroluminescent display device according to the related art. As shown in FIG. 4, an organic electroluminescent display device may produce a chess-board image having white and black images. In particular, the chess-board image includes a white region “A” and a black region “D.” Thus, this chess-board image is produced by applying the third voltage “V3” (as shown in FIG. 3) to the gate electrode of the driving TFT in the white region “A” and by applying the first voltage “V1” (as shown in FIG. 3) to the gate electrode of the driving TFT in the black region “D.”
Ideally, the organic electroluminescent display device should produce gray images of an equal brightness throughout all display areas when a same level of driving voltage is applied thereto, even immediately after the chess-board pattern is displayed. However, when the driving TFT has a hysteresis as shown in FIG. 3, a current value of the driving TFT in the white region “A” is different from a current value of the driving TFT in the black region “D.” For example, as shown in FIG. 4B, when the organic electroluminescent display device is driven to display a gray image over its entire display area immediately after a chess-board image is displayed, a resultant image includes a first gray region “B” and a second gray region “C” respectively corresponding to the white region “A” and the black region “D.” In particular, a brightness of the first gray region “B” is lower than a brightness of the second gray region “C.”
That is, although the same second voltage V2 is applied to the gates electrodes of the driving TFTs in the first and second gray regions “B” and “C,” the driving TFT in the first gray region “B” has the third current value “IC” and the driving TFT in the second gray region “C” has the second current value “IB.” In particular, the second current value “IB” is higher than third current value “IC” because the driving TFT has a hysteresis as shown in FIG. 3. Accordingly, the hysteresis of the driving TFT causes an abnormal display such as a gray chess image and an residual image as shown in FIG. 4B.